Fix cleanup in lib/libc/gen/setdomainname_test

ATF cleanup routines run in separate processes from the tests themselves, so
they can't share global variables.

Also, setdomainname_test needs to be is_exclusive because the test cases
access a global resource.

PR: 219967
Reviewed by: ngie
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D11188